The capacitive signals of capacitive carbon nanotube humidity sensors are usually tested by measuring instruments, which restricts the uses and developments of the sensors seriously. Focusing on this problem，one kind of capacitive carbon nanotube humidity sensor based on a 555 multivibrator was presented. Firstly, the principle of the sensor and the detecting circuit were discussed, and the sensor was designed and fabricated. Then, the sensor was tested by both the RCL apparatus and the proposed circuit, respectively. Finally, the response and recovery time of the sensor was tested. The testing results demonstrate that when the Relative Humidities (RH) surrounding the sensor change from 11% to 97%, the capacitive sensitivity and the output frequency sensitivity of the sensor are 905% and 889%,respectively. The frequency sensitivity is closed to the capacitive sensitivity, which means the circuit can well change capacitive signals to frequency signals.Furthermore, the adesorption and desorption time of the sensor is about 4 s and 18 s, respectively,which shows the sensor can offer low power consumption and fast response and has potential application prospects.